Use of implantable medical devices (IMDs) is ubiquitous in treating cardiac diseases. These IMDs analyze cardiac electrical activity of a patient to monitor and assess their cardiac condition. For example, many patients who are at risk of cardiac ischemia have pacemakers or implantable cardioverter/defibrillators (ICDs). These IMDs sense the cardiac activity of the patient and stimulate the patient's heart by providing electrical pulses to restore the heart rate to a normal rhythm. Oftentimes, once the IMD detects an abnormal cardiac activity, the IMD takes an action to correct the abnormal incident either by sending pulses to the patient's heart or alerting a clinician or dispending a drug if applicable.
Many of these devices analyze various characteristics of electrocardiogram (ECG), such as the T wave and ST segments. However, there exist certain drawbacks when using these characteristics of ECG for monitoring the overall cardiac condition of a patient. First, such data acquired from an implantable device may not be equivalent to data acquired using external ECG electrodes (e.g., during “routine” cardiac stress testing or T wave alternans testing). As such, clinical interpretation based upon data acquired through an implantable device may be misleading if such interpretation is referenced to knowledge garnered from studies involving externally acquired data (i.e. using surface ECG electrodes). For example, 2 microvolts of T wave alternans obtained through an internal device is, in fact, known to be not equivalent to 2 microvolts of T wave alternans measured through surface ECG electrodes. The amplitude of measured alternans increases as one goes below the surface of the skin to measure it. Moreover, internally acquired ECG signals through an implantable device suffer from limitations relating to the fact that the number of discrete ECG vectors that can be used are severely limited in comparison to measurement using external ECG electrodes. This is an intrinsic limitation of every approach that seeks to use internally measured ECG phenomenon (such as T wave alternans and ischemia) as the principal method of diagnosis. Since it is well known that such phenomenon may be seen in one vector but not another, and since the range of vector analysis through an internal device is more limited than using the external electrodes in which multiple vectors are used, diagnosis by external electrodes (assuming equivalent sensitivity for each vector) is preferred as it will result in less “under-detection”. Therefore, making a clinical diagnosis based on external ECG measurement may reduce under-detection of cardiac diseases.
As shown, the implantable devices which use internally detected ECG phenomenon (e.g. T wave alternans and ST segment changes) as the principal means for detecting clinically significant physiological processes (e.g. ventricular arrhythmic substrate and cardiac ischemia) are intrinsically limited in that they: a) may under-detect the phenomenon or physiological process of interest, b) may produce an output result that is misleading because it is not equivalent to the same result obtained using traditional ECG electrodes which clinicians are more familiar with and for which there is much more clinical data to guide therapy.
Some implantable devices contain software that detects heart rate (HR) and/or pathologic cardiac rhythms (i.e. cardiac arrhythmias), and subsequently delivers this information to clinicians. However, while these devices are able to detect and even treat these pathologic heart rhythms, they do not guide the physician along a clinical strategy for preventing these cardiac arrhythmias in the first place—they are reactive rather than preventative. This latter fact exposes patients to greater risk than a strategy that prevents arrhythmia (or ischemia) in the first place. The fact that T wave alternans and cardiac ischemia may both be heart rate dependant phenomenon (i.e. become present at higher heart rates) makes heart rate monitoring a useful approach to prevent and/or diagnose those conditions.